This application relates to superconductors and more particularly to low temperature superconductors.
Multifilamentary Low Temperature Superconduct or (LTS) cables are normally produced by drilling bronze cylinders, inserting rods of superconductor precursor wire, drawing down to fine diameter and heat treating to obtain the superconducting properties. Unfortunately, a similar procedure cannot be used in the case of aluminum because of its tenacious surface oxide layer. For example, pure aluminum ribbon may be wrapped on Nb-Ti wire and sections of same may be inserted into a tube and swaged. There remains, however, present on the aluminum surface its oxide layer, which is very difficult to dislodge. Thus when individual strands are consolidated in a tube, the oxide layers remain in between inhibiting strong bonding. This, in turn, cause poor thermal and electrical characteristics of the cable.
Another fabrication problem encountered in drawing or swaging multifilamentary wires within a soft metal surrounding matrix is "sausaging."This is evidenced as a pinching off of individual strands as the composite wire is swaged. Swaging aluminum Ion Vapor Deposited (IVD) Nb-Ti wire strands has shown that sausaging largely can be eliminated if the soft aluminum layer on the strands is thin and uniform. A working criterion for this is that the aluminum coating be very much less than the wire diameter The thin aluminum layer is very difficult to obtain by wrapping Harder aluminum alloys must be used, which may not produce as acceptable a superconductor cable.
Finally, if the Nb-Ti strands are simply dipped into molten aluminum, a ring of reaction products forms in the outer portion of the wire/aluminum interface. This would be detrimental to the final quality of multistrand cable made from this wire as evidenced in reduced critical current (J.sub.c) and poor thermal stability.